The modification of proteins with additional agents such as polymers has commonly been employed for the purpose of providing improved pharmaceutical properties, e.g. improved stability and retention in the blood and reduced antigenicity [for example, see F. M. Veronese and J. M. Harris, “Peptide and Protein Pegylation”, Advanced Drug Delivery Reviews 54(4), 2002].
When modifying proteins with a polymer, one technique employed in the past comprises binding the protein and the polymeric modifier via a covalent bond (e.g. see WO-A-97/23614). In other examples of the polymer modification of a protein, such as in the modification of a drug with a polymer, the drug has been modified via a non-covalent bond. One such example is provided in Japanese Patent Application (Kokai) No. Hei 11-302199, which discloses that a graft copolymer, which comprises a graft chain of a non-ionic polymer and a main chain of a negatively-charged polymer, forms an inclusion complex with a substance capable of being positively charged under physiological conditions, for example, a liposome or poly-L-lysine carrying positive charge, to improve the retention in blood. Another alternative is suggested in WO-A-99/02131 which discloses that a protein and a water-soluble polymer can be mixed under specific conditions in the presence of an organic solvent to provide controlled-release microparticles.
Unfortunately, few of these polymeric protein modifiers have been particularly successful for a variety of reasons. One recent example of a polymeric protein modifier that shows some improved properties comprises a polymaleic acid compound-based copolymer containing a polyoxyalkylene alkyl ether compound as a constituent unit [see, for example, Japanese Patent Nos. 3035675 and 3271265]. These polymeric modifiers certainly show improved binding to the target proteins. However, significant problems still exist with such copolymers. The maleic anhydride moiety thereof is found to bind to proteins non-specifically. This results in the obtained complexes of the copolymer and the protein showing non-uniform properties depending upon the conditions. Particularly, it is found that these polymeric modifiers tend to readily form disorganised cross-linked structures with the proteins thus forming bulky complexes that cause excessive modification of the protein structure and hence reduction of desired protein activity. Furthermore, complexes of these polymeric protein modifiers and a protein have been found to show unsatisfactory retention in the blood after administration.
There is, therefore, a need for a polymeric modifier capable of providing a complex having uniform properties, especially reduced production of disorganised cross-linked structures with the protein, better maintenance of protein activity and excellent retention of the protein in the blood after administration of said complex.